Unified approach for holography and shearography in surface deformation measurement and nondestructive testing

Holography and shearography are two useful whole-field non- contacting optical tools for nondestructive flaw detection and precision measurements. Holography serves as a displacement transducer since it gives direct measurements on displacements whereas shearography serves as a strain gage since it gives direct measurements on displace- ment gradients. This paper views holography and shearography and their variations as a single optical technique having the same basic mathematical formulation and instrumentation. A key optical component used in both techniques is a doubly-refractive prism that combines two angularly separated laser rays to interfere at near collinearity, thereby permitting the use of a low-resolution CCD camera for recording the interference pattern. Shearography uses a doubly-refractive prism with small image shearing so that two neighboring points on the test surface are brought to interfere at the image plane of the camera, whereas ho- lography, on the other hand, uses a doubly-refractive prism with large image shearing so that light scattered from two different objects—a test object and a reference surface (serving as a reference beam)—are brought to interfere at the image plane of the camera. Hence, testing and measurements made using holography may also be made using shearography, and vice versa. © 2003 Society of Photo-Optical Instrumentation

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